Ion Conducting Polymer Blend Electrolyte Research Articles

Chitosan as a suitable host for sustainable plasticized nanocomposite sodium ion conducting polymer electrolyte in EDLC applications: Structural, ion transport and electrochemical studies

The challenge in front of EDLC device is their low energy density compared to their battery counter parts. In the current study, a green plasticized nanocomposite sodium ion conducting polymer blend electrolytes (PNSPBE) was developed by incorporating plasticized Chitosan (CS) blended with polyvinyl alcohol (PVA), doped with NaBr salt with various concentration of CaTiO3 nanoparticles. The most optimized PNSPBE film was subsequently utilized in an EDLC device to evaluate its effectiveness both as an electrolyte and a separator. Structural and morphological changes were assessed using XRD and SEM techniques. The PNSPBE film demonstrated a peak ionic conductivity of 9.76×10−5 S/cm, as determined through EIS analysis. The dielectric and AC studies provided further confirmation of structural modifications within the sample. Both TNM and LSV analyses affirmed the suitability of the prepared electrolyte for energy device applications, evidenced by its adequate ion transference number and an electrochemical potential window of 2.86 V. Electrochemical properties were assessed via CV and GCD techniques, confirming non-Faradaic ion storage, indicated by the rectangular CV pattern at low scan rates. The parameters associated with the designed EDLC device including specific capacitance, ESR, power density (1950 W/kg) and energy density (12.3 Wh/kg) were determined over 1000 cycles.

The study of ion transport parameters using two models in plasticized sodium ion conducting polymer blend electrolytes

The study of ion transport parameters using two models in plasticized sodium ion conducting polymer blend electrolytes

Dielectric properties and ion transport studies from Trukhan, EIS, and Bandara-Mellander approaches for plasticized sodium ion conducting polymer blend electrolytes

Dielectric properties and ion transport studies from Trukhan, EIS, and Bandara-Mellander approaches for plasticized sodium ion conducting polymer blend electrolytes

Microstructure and electrical properties of Li+ ion conducting polymer blend electrolyte films

Microstructure and electrical properties of Li+ ion conducting polymer blend electrolyte films

Synthesis of Solid Polymer Blend Electrolytes Based on PVP/PAN/Ammonium Thiocyanate: Structural, Electrical, Dielectric and Electrochemical Analysis

In this work, a solution casting technique was used to prepare stable and free-standing films of blend polymer electrolytes composed of polyvinyl pyrrolidone (PVP) and polyacrylonitrile (PAN) complexed with ammonium thiocyanate (NH4SCN) salt in various compositions. The physical characterization by X-ray diffraction (XRD) was performed to study the structural properties of the samples. Characteristics of these electrolytes were also examined using impedance technique, dielectric studies, transport number measurement (TNM) and linear sweep voltammetry (LSV). The results showed that cationic transport predominated in highly amorphous films. Impedance spectroscopy was utilized to determine the dependence of electrical properties on both composition and temperature. The temperature dependence of the electrical conductivity showed behaviour similar to that of Arrhenius type. It was found that the highest ion conducting polymer blend electrolyte led to lower activation energy.

Fabrication and Characterization of Flexible Solid Polymers Electrolytes for Supercapacitor Application.

In this work, solid flexible polymer blend electrolytes (PBE) composed of polyvinyl alcohol (PVA) and polyvinyl pyrrolidone (PVP) with different amounts of sodium thiocyanate (NaSCN) salt mixed in double-distilled water (solvent) are prepared via solution casting method. The obtained films are characterized using several techniques. The study of the surface morphology of the polymer blend salt complex films via the POM technique reveals the presence of amorphous regions due to the NaSCN effect. FTIR spectra studies confirm the complex formation between PVA, PVP, and NaSCN. The addition of 20 wt% NaSCN salt in the composition PVA: PVP (50:50 wt%) polymer blend matrix leads to an increase in the number of charge carriers and thus improves the ionic conductivity. The ionic conductivity of each polymer blend electrolyte was studied using the electrochemical impedance spectroscopy (EIS) method. The highest room temperature ionic conductivity of 8.1 × 10−5 S/cm S cm−1 is obtained for the composition of PVA: PVP (50:50 wt%) with 20 wt% NaSCN. LSV test shows the optimized ion-conducting polymer blend electrolyte is electrochemically stable up to 1.5 V. TNM analysis reveals that 99% of ions contribute for the conductivity against 1% of electrons only in the highly conductive polymer electrolyte PVA: PVP (50:50 wt%) + 20 wt% NaSCN. A supercapacitor device was fabricated using the optimized ion-conducting polymer blend film and graphene oxide (GO) coated electrodes. The GCD curve clearly reveals the behavior of an ideal capacitor with less Faradic process and low ESR value. The columbic efficiency of the GO-based system is found to be 100%, the GO-based electrode exhibits a specific capacitance of 12.15 F/g and the system delivers the charge for a long duration. The specific capacitance of the solid-state supercapacitor cell was found to be 13.28 F/g via the CV approach close to 14.25 F/g obtained with EIS data at low frequency.

The H+ ion transport study in polymer blends incorporated with ammonium nitrate: XRD, FTIR, and electrical characteristics

The present study gained insights into H+ ion transport through non-destructive techniques. The XRD outcome reveals that poly(2-ethyl-2-oxazoline) (POZ) reduced the crystalline phase of chitosan (CS) effectively. Films of ion-conducting polymer blend electrolytes were created from CS:POZ complexed with ammonium nitrate (NH4NO3) as an H+ supplier by using the solution casting method. X-ray diffraction was used to study the structural alterations that occurred. The degrees of crystallinity were estimated from the deconvoluted XRD pattern. Measurement of electrical impedance was carried out for the electrolytes. From FTIR measurement, free ion and ion pair values were calculated. Several ion transport parameters (ITP), such as number density (n), ionic mobility (μ), and diffusion coefficient (D), were determined. It was found that the maximum conductivity was 3.87 × 10−6 S cm−1 at 30 percent NH4NO3. The DC conductivity pattern changes linearly with temperature. Dielectric and electric modulus were studied in detail. It was discovered that the loss tangent peak transferred to a upper side of frequency when the salt concentration was increased. Long-range conductivity relaxation was seen in the Mi spectra. The lack of a full semicircle arc on the Argand figure suggested non-Debye ion dynamics. The observed AC conductivity (σac) spectra showed three different zones. In the high-frequency range, the σac value follows a power law (σac=Aωs).

Development of Flexible Plasticized Ion Conducting Polymer Blend Electrolytes Based on Polyvinyl Alcohol (PVA): Chitosan (CS) with High Ion Transport Parameters Close to Gel Based Electrolytes.

In the current study, flexible films of polyvinyl alcohol (PVA): chitosan (CS) solid polymer blend electrolytes (PBEs) with high ion transport property close enough to gel based electrolytes were prepared with the aid of casting methodology. Glycerol (GL) as a plasticizer and sodium bromide (NaBr) as an ionic source provider are added to PBEs. The flexible films have been examined for their structural and electrical properties. The GL content changed the brittle and solid behavior of the films to a soft manner. X-ray diffraction (XRD) and Fourier transform infrared (FTIR) methods were used to examine the structural behavior of the electrolyte films. X-ray diffraction investigation revealed that the crystalline character of PVA:CS:NaBr declined with increasing GL concentration. The FTIR investigation hypothesized the interaction between polymer mix salt systems and added plasticizer. Infrared (FTIR) band shifts and fluctuations in intensity have been found. The ion transport characteristics such as mobility, carrier density, and diffusion were successfully calculated using the experimental impedance data that had been fitted with EEC components and dielectric parameters. CS:PVA at ambient temperature has the highest ionic conductivity of 3.8 × 10 S/cm for 35 wt.% of NaBr loaded with 55 wt.% of GL. The high ionic conductivity and improved transport properties revealed the suitableness of the films for energy storage device applications. The dielectric constant and dielectric loss were higher at lower frequencies. The relaxation nature of the samples was investigated using loss tangent and electric modulus plots. The peak detected in the spectra of tanδ and M” plots and the distribution of data points are asymmetric besides the peak positions. The movements of ions are not free from the polymer chain dynamics due to viscoelastic relaxation being dominant. The distorted arcs in the Argand plot have confirmed the viscoelastic relaxation in all the prepared films.

The Study of Plasticized Sodium Ion Conducting Polymer Blend Electrolyte Membranes Based on Chitosan/Dextran Biopolymers: Ion Transport, Structural, Morphological and Potential Stability.

The polymer electrolyte system of chitosan/dextran-NaTf with various glycerol concentrations is prepared in this study. The electrical impedance spectroscopy (EIS) study shows that the addition of glycerol increases the ionic conductivity of the electrolyte at room temperature. The highest conducting plasticized electrolyte shows the maximum DC ionic conductivity of 6.10 × 10−5 S/cm. Field emission scanning electron microscopy (FESEM) is used to investigate the effect of plasticizer on film morphology. The interaction between the electrolyte components is confirmed from the existence of the O–H, C–H, carboxamide, and amine groups. The XRD study is used to determine the degree of crystallinity. The transport parameters of number density (n), ionic mobility (µ), and diffusion coefficient (D) of ions are determined using the percentage of free ions, due to the asymmetric vibration (υas(SO3)) and symmetric vibration (υs(SO3)) bands. The dielectric property and relaxation time are proved the non-Debye behavior of the electrolyte system. This behavior model is further verified by the existence of the incomplete semicircle arc from the Argand plot. Transference numbers of ion (tion) and electron (te) for the highest conducting plasticized electrolyte are identified to be 0.988 and 0.012, respectively, confirming that the ions are the dominant charge carriers. The tion value are used to further examine the contribution of ions in the values of the diffusion coefficient and mobility of ions. Linear sweep voltammetry (LSV) shows the potential window for the electrolyte is 2.55 V, indicating it to be a promising electrolyte for application in electrochemical energy storage devices.

The Study of Plasticized Solid Polymer Blend Electrolytes Based on Natural Polymers and Their Application for Energy Storage EDLC Devices.

In this work, plasticized magnesium ion-conducting polymer blend electrolytes based on chitosan:methylcellulose (CS:MC) were prepared using a solution cast technique. Magnesium acetate [Mg(CH3COO)2] was used as a source of the ions. Nickel metal-complex [Ni(II)-complex)] was employed to expand the amorphous phase. For the ions dissociation enhancement, glycerol plasticizer was also engaged. Incorporating 42 wt% of the glycerol into the electrolyte system has been shown to improve the conductivity to 1.02 × 10−4 S cm−1. X-ray diffraction (XRD) analysis showed that the electrolyte with the highest conductivity has a minimum crystallinity degree. The ionic transference number was estimated to be more than the electronic transference number. It is concluded that in CS:MC:Mg(CH3COO)2:Ni(II)-complex:glycerol, ions are the primary charge carriers. Results from linear sweep voltammetry (LSV) showed electrochemical stability to be 2.48 V. An electric double-layer capacitor (EDLC) based on activated carbon electrode and a prepared solid polymer electrolyte was constructed. The EDLC cell was then analyzed by cyclic voltammetry (CV) and galvanostatic charge–discharge methods. The CV test disclosed rectangular shapes with slight distortion, and there was no appearance of any redox currents on both anodic and cathodic parts, signifying a typical behavior of EDLC. The EDLC cell indicated a good cyclability of about (95%) for throughout of 200 cycles with a specific capacitance of 47.4 F/g.

The Study of Electrical and Electrochemical Properties of Magnesium Ion Conducting CS: PVA Based Polymer Blend Electrolytes: Role of Lattice Energy of Magnesium Salts on EDLC Performance.

Plasticized magnesium ion conducting polymer blend electrolytes based on chitosan (CS): polyvinyl alcohol (PVA) was synthesized with a casting technique. The source of ions is magnesium triflate Mg(CF3SO3)2, and glycerol was used as a plasticizer. The electrical and electrochemical characteristics were examined. The outcome from X-ray diffraction (XRD) examination illustrates that the electrolyte with highest conductivity exhibits the minimum degree of crystallinity. The study of the dielectric relaxation has shown that the peak appearance obeys the non-Debye type of relaxation process. An enhancement in conductivity of ions of the electrolyte system was achieved by insertion of glycerol. The total conductivity is essentially ascribed to ions instead of electrons. The maximum DC ionic conductivity was measured to be 1.016 × 10−5 S cm−1 when 42 wt.% of plasticizer was added. Potential stability of the highest conducting electrolyte was found to be 2.4 V. The cyclic voltammetry (CV) response shows the behavior of the capacitor is non-Faradaic where no redox peaks appear. The shape of the CV response and EDLC specific capacitance are influenced by the scan rate. The specific capacitance values were 7.41 F/g and 32.69 F/g at 100 mV/s and 10 mV/s, respectively. Finally, the electrolyte with maximum conductivity value is obtained and used as electrodes separator in the electrochemical double-layer capacitor (EDLC) applications. The role of lattice energy of magnesium salts in energy storage performance is discussed in detail.

Structural, electrical and electrochemical studies of sodium ion conducting blend polymer electrolytes

In the present study sodium ion conducting polymer blend electrolytes has been prepared using poly (vinylidene fluoride – hexafluoro – propylene) (PVdF-HFP), poly (methyl methacrylate) (PMMA), and sodium thiocyanate (NaSCN) salt by solution-cast technique. The highest ionic conductivity of the optimized blend polymer electrolyte system [PVdF(HFP)-PMMA (4:1)] (20 wt%)-[NaSCN (1 M)] (80 wt%) has been found to be 4.54 × 10−2 S cm−1 at room temperature. The temperature dependence conductivity plot shows the Arrhenius behaviour and its activation energy calculated from the plot were found to be 0.13 eV. The structural and morphological studies of polymer blend electrolyte were investigated by XRD, SEM and FTIR spectroscopy. Complex formation between polymer and salt has been confirmed by these studies. The thermal properties of optimized electrolyte system were examined by differential scanning calorimetry (DSC) techniques. The ionic transport number was calculated using d.c polarization techniques and was found to be 0.92, which shows that electrolyte system is predominantly ionic in nature. The electrochemical potential window of optimized polymer blend electrolyte was tested and observed to be ∼2.8 V.

Investigations of lithium ion conducting polymer blend electrolytes using biodegradable cornstarch and PVP

Solid polymer electrolytes used in energy storage devices based on Cornstarch and Poly (vinyl Pyrrolidone) blend incorporated with lithium acetate was prepared using the solution casting method. XRD results confirmed that the amorphous nature of the as-prepared polymer electrolyte was increased by addition of salt. 80 wt % Cornstarch/20 wt % PVP/60 wt % lithium acetate added polymer blend system attains better amorphous nature than others. At ambient temperature, 60 wt % of lithium acetate added system has better bulk conductivity of 3.52 × 10−5Scm−1 over the frequency range of 42 Hz to 1 MHz using AC impedance spectroscopy studies. The dielectric analysis is confirmed that the higher conducting sample was having high dielectric constant and low relaxation time for enhancing ions dynamic behaviour. The conduction mechanism of the higher conducting polymer electrolyte is explained by small polaron hopping and overlapping large polaron tunnelling conduction mechanism. From the argand plot, it is confirmed that higher conducting sample has low relaxation time.

Studies on Conducting Polymer Blends: Synthesis and Characterizations of PVA/PVP Doped with CaCl₂.

Calcium ion conducting polymer blend electrolyte films based on PVA/PVP complexed with CaCl2 have been prepared by solution casting technique. The structural changes of the polymer blends have been studied by X-ray diffraction (XRD) and Fourier transform infrared (FT-IR) spectroscopy. XRD and FT-IR studies confirmed the complex formation of polymer blends. Electrical conductivity was calculated with impedance analyzer within the frequency range 42 Hz-1 MHz and in the temperature range 303 K-340 K. The higher electrical conductivity value of 1.704 × 10-4 Scm-1 was observed for 50PVA:50PVP:15 wt% CaCl2 concentration at room temperature. The magnitude of electrical conductivity was increased with the increase in the salt concentration as well as temperature. The electrical permittivity of the polymer films have been studied for various temperatures.

Characterization of Lithium Ion-Conducting Blend Biopolymer Electrolyte Based on CH–MC Doped with LiBF4

Lithium ion-conducting polymer blend electrolytes based on chitosan and methylcellulose complexed with lithium tetrafluoroborate (LiBF4) were prepared by a solution-casting method. The features of complexation of the solid polymer electrolytes were studied using X-ray diffraction techniques. Electrical conductivity of the prepared films was measured as a function of frequency at a different temperature. The increased trend of the electrical conductivity with increasing temperature and salt concentration can be attributed to increasing the mobility and number of lithium ions, respectively. The polymer electrolyte system exhibited Arrhenius-type, temperature-dependence ion conductivity behavior. Optical properties such as optical band gap, tail due to localized states and complex refractive index were estimated for present polymer electrolyte system from optical absorption measurement in the wavelength region 190–1100 nm. It was found that the optical direct band gap values shifted to lower energies upon addition of LiBF4 salt up to 40 wt% dopant concentration, and showed an increasing tendency for a further increase in dopant concentration. The high refractive index for this composition (2.44–2.63) at visible wavelengths eminently suitable for optical applications.

Study of PVAc-PMMA-LiCl polymer blend electrolyte and the effect of plasticizer ethylene carbonate and nanofiller titania on PVAc-PMMA-LiCl polymer blend electrolyte

Abstract lithium ion conducting polymer electrolyte is one of the essential components of modern rechargeable lithium batteries because of its good interfacial contact with electrodes and effective mechanical properties. A solid lithium ion conducting polymer blend electrolyte is prepared using poly (vinyl acetate) (PVAc) and poly (methyl methacrylate) (PMMA) polymers with different molecular weight percentages (wt%) of lithium chloride (LiCl) by the solution casting technique with tetrahydrofuran as a solvent. The polymer electrolytes were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), Thermogravimetry (TG), AC impedance spectroscopy and ionic transport measurements. XRD and FTIR studies confirm the amorphous nature of the polymer electrolyte and the complexation of salt with polymer. The thermal behavior of polymer electrolytes has been studied from DSC and TG. The highest conductivity obtained using AC impedance spectroscopy is 1.03×10−5 Scm−1 at 303 K for 70 wt%PVAc:30 wt%PMMA:0.8 wt% of LiCl polymer-salt complex. The plasticizer ethylene carbonate (EC) and nanofiller titania (TiO2) were added to the optimized high conducting blend polymer electrolyte. An enhancement in conductivity by one order of magnitude was observed for the plasticized 70 wt%PVAc-30 wt%PMMA-0.8 wt% LiCl polymer electrolyte at ambient temperature. The ionic conductivity value obtained using AC impedance spectroscopy for the plasticized 70 wt%PVAc-30 wt%PMMA-0.8 wt% LiCl polymer electrolyte was 1.03×10−4 Scm−1. The highest conductivity obtained for 70 wt%PVAc-30 wt%PMMA-0.8% LiCl-6 mg TiO2 was 4.45×10−4 Scm−1. Dielectric properties of polymer films are studied and discussed. The electrochemical stability of 1.69 V and 2.69 V was obtained for 70 wt%PVAc-30 wt%PMMA-0.8% LiCl and 70 wt%PVAc-30 wt%PMMA-0.8% LiCl-6 mg TiO2 polymer electrolytes, respectively, using linear sweep voltammetry. The value of Li+ ion transference number was estimated by the DC polarization method and was found to be 0.99 for the highest conducting 70 wt%PVAc-30 wt%PMMA-0.8 wt% LiCl-6 mg TiO2 nanocomposite polymer electrolyte.

Impedance spectroscopy, ionic conductivity and dielectric studies of new Li+ ion conducting polymer blend electrolytes based on biodegradable polymers for solid state battery applications

New solid polymer blend electrolyte films based on biodegradable polymer blend comprising of polyvinyl alcohol (PVA) and poly (N-vinyl pyrrolidone) (PVP) doped with different wt% of lithium carbonate (Li2CO3) salt have been prepared by solution casting method. The resulting PVA/PVP/Li2CO3 polymer blend electrolyte films have been characterized by various analytical techniques such as FTIR, UV–vis, XRD, TGA, polarized optical microscopy and scanning electron microscopy. The FTIR and XRD analysis confirmed the complex formation between PVA/PVP blend and Li2CO3 salt. The ionic conductivity and the dielectric properties of PVA/PVP/Li2CO3 polymer blend electrolyte films were investigated using an impedance spectroscopy. It was observed that the ionic conductivity of PVA/PVP/Li2CO3 electrolyte system increases as a function of Li2CO3 concentration. The highest ionic conductivity was found to be 1.15 × 10−5 S cm−1 for polymer blend electrolyte with 20 wt% Li2CO3 content. On the other hand, the dielectric results revealed the non-Debye type of behaviour. The dielectric constant values indicate a strong dielectric dispersion in the studied frequency range which increases as the Li2CO3 content increases. The dielectric constant as high as 1200 (e = 1201.57, 50 Hz, 150 °C) and the dielectric loss well below 4 (tan δ = 3.94, 50 Hz, 150 °C) were obtained for polymer blend electrolytes with 25 wt% Li2CO3 salt. Thus, the results obtained in the present study suggest that the PVA/PVP/Li2CO3 polymer blend electrolyte system seems to be a promising candidate for solid state battery applications.

Investigations on PEO/PVP/NaBr complexed polymer blend electrolytes for electrochemical cell applications

Experimental investigations on a sodium ion conducting polymer blend electrolyte system based on polyethylene oxide (PEO) and polyvinyl pyrrolidone (PVP), complexed with NaBr salt are presented in this paper. The complexed polymer blend electrolytes were obtained in the form of dimensionally stable and free-standing films by solution cast technique. Physical characterization by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were performed to study the structural properties, their change along with ion–polymer interactions and thermal stability of the samples. The transport numbers of different mobile species were determined by means of d.c. polarization and combined a.c./d.c. techniques. Cationic transportation was found to be dominant in the films of high degree of amorphicity. Electrical properties were measured as a function of composition and temperature using complex impedance spectroscopy. The temperature dependent electrical conductivity showed Arrhenius type behavior. Activation energies were found to decrease with increasing concentration of the salt. Electrochemical cells were prepared using PEO/PVP/NaBr polymer blend electrolyte system and their discharge characteristics were studied. These studies suggested the practical realization of electrochemical cells using the present sodium ion conducting polymer blend electrolyte system.

AC conduction mechanism and battery discharge characteristics of (PVC/PEO) polyblend films complexed with potassium chloride

New K+ ion-conducting polymer blend electrolyte films based on amorphous polymer poly(vinyl chloride) (PVC) and semicrystalline polymer poly(ethylene oxide) (PEO) complexed with KCl salt were prepared using a solution-cast technique. The maximum value of ionic conductivity of a PVC/PEO:KCl (42.5:42.5:15) system is 8.29×10−6S/cm at 303K. The absorption edge was found at 4.30eV for undoped film, while it was observed at 4.03 and 3.93eV for 10 and 15wt% KCl doped films, respectively. The direct band gaps for these pure and salt doped PVC/PEO films were found to 4.10, 3.86 and 3.74eV, respectively, whereas the indirect band gaps were determined as 4.15, 3.72 and 3.64eV. Transference number values showed that the charge transport in this electrolyte system is predominantly due to ions (tion=0.97). The discharge capacity for PVC/PEO:KCl (42.5:42.5:15) polymer blend electrolyte system is 11μA/h.

Preparation and characterization of pure and KBr doped polymer blend (PVC/PEO) electrolyte thin films

A potassium ion conducting polymer blend electrolyte based on polyvinyl chloride (PVC) and polyethylene oxide (PEO) complexed with KBr was prepared using solution cast technique. The structural properties of these electrolyte films were examined by XRD studies. The XRD data revealed that the amorphous domains of polymer blend matrix increased with increase of KBr salt concentration. The complexation of the salt with the polymer blend was confirmed by FT-IR studies. Electrical conductivity of the films was measured with impedance analyzer in the frequency range of 100Hz to 1MHz and in the temperature range of 303–363K. It was observed that the magnitude of conductivity increased with the increase in the salt concentration as well as the temperature. UV–VIS absorption spectra in wavelength region of 200–600nm were used to evaluate the optical properties like direct band gap, indirect band gap and absorption edge. The optical band gaps decreased with increasing of K+ ion concentration. This suggests that, KBr is a good dopant to improve the electrical properties of PVC+PEO polymer blend electrolytes. Studies on the electrical and optical properties of polymer blend electrolyte have attracted much attention in view of their application in electronic and optical devices like electro-chromic display devices, fuel cells, gas sensors and solid state batteries. The thermal properties of these films were investigated with differential scanning calorimetry (DSC). The variation in film morphology is examined by scanning electron microscopy examination (SEM).